Athermally stable laser device

ABSTRACT

Laser light generator containing a glass laser rod with a composition so chosen that the net thermal effects on the index of refraction are negated. The algebraic sum of the effects of the change in index of refraction caused by the temperature coefficient of the index of refraction, the coefficient of linear expansion, Poisson&#39;&#39;s ratio and the stress-optical effects, for light in one plane of polarization is chosen to be near zero, while remaining small but negative for the other polarization component.

United States Patent [451 Oct. 17, 1972 [73] Assignee: American OpticalCorporation, Southbridge, Mass.

[22] Filed: March 10, 1970 [211 App]. No.: 18,253

[52] US. Cl. ..33l/94.5, 330/4.3, 252/30l.4 F, 252/30l.6 F, 106/52 [51]Int. Cl ..H0ls 3/16 [58] Field of Search....252/30l.4 F, 301.6 F;106/52; 331/945; 330/43 3,384,597 5/1968 Paolis et al. ..252/301.6 F3,422,025 l/l969 Snitzer et al ..252/301 .6 F 3,528,927 9/1970 Graf..252/30l.6 F

Primary Examiner-James E. Poer Assistant Examiner-J. CooperAttorney-Lane, Aitken, Dunner & Ziems and William C. Nealon [57]ABSTRACT Laser light generator containing a glass laser rod with acomposition so chosen that the net thermal effects on the index ofrefraction are negated. The algebraic sum of the effects of the changein index of refraction caused by the temperature coefficient of theindex of refraction, the coefficient of linear expansion, Poisson'sratio and the stress-optical effects, for light in one plane ofpolarization is chosen to be near zero, while remaining small butnegative for the other polarization component.

10 Claims, 3 Drawing Figures PATENTEUUCT 17 I972 3,699 .472

0 FIG. I.') l (PRIOR ART 2O INVENTOR CHARLES GILBERT YOUNG AT RNEYSATHERMALLY STABLE LASER DEVICE BACKGROUND OF THE INVENTION The field ofthis invention is lasers and more particularly glass laser materialswhich reduce the effects of thermal gradients within the laser material.

It is known that if the only light allowed to reflect bidirectionallythrough the cavity of a laser light generator is light emitted in theaxial plane wave modes, a high degree of emissive efficiency isachieved. This desired result is accomplished by limiting stimulatedemission predominantly to mode-selected plane wave light energy.

The laser output of light in the plane wave front which is the usefulportion of the output pulse is significantly greater for laser lightresulting essentially from bidirectional reflection of axial light thanit is when bidirectional reflection of light in off-axis modes isallowed to develop in the cavity. With predominantly axial light beingthe only light reflected, the beam spread angle of the output pulse isreduced, and as a result the output intensity, or power per unit solidangle delivered by the laser at any given distance (an inverse functionof the beam spread angle), is advantageously increased. These samemode-selective considerations are useful with all laser generators, thatis, amplifiers as well as oscillators.

In laser applications it is desirable to maintain the beam spread angleat a low value. As is pointed out above, this can be accomplished bymode selection which limits the modes propagating and reflecting withinthe laser cavity to axial modes. This type of mode selection has beenaccomplished through mechanical means. Laser devices including suchmechanical mode selection means are suitable to produce a laser outputwith a small beam spread angle provided the laser is not operated atrepetition rates sufficiently high so that succeeding shots occur beforecomplete cooling results from prior shots. With high repetition ratesof, for example, about one pulse per minute (ppm) for a 4 cm-diameterrod or about one pulse per second (pps) for a 6 millimeter-diameter rod,it is necessary to consider the thermal effects created by pumping thelaser rod if the beam spread angle is to be maintained at a minimum.

Pumping is accomplished by a flash tube, as it is called, which providesthe initial energy inversion. The flash tube may be in the form of ahelix concentrically surrounding and in spatial relationship to thelaser rod, with coils that are equally spaced along the length of thelaser rod to distribute its heat emission evenly along the length of therod. However, the radial heat distribution is quite uneven with theflash tube causing different temperatures at the axis of the rod than atthe periphery. The thermal stress distribution in the rod is, therefore,similarly uneven, causing an index of refraction gradient from thecenter to the edge of the rod during pumping by the flash tube and aresulting reduction in beam definition which is intimately related to adesired laser output. During operation of laser devices cooling-gradientinduced index changes also occur which cause similar if not more seriousproblems than pumping-gradient induced index changes. Also, the sametype of temperature variations are produced with other flash tubes suchas those in cylindrical form with the cylinder axis parallel andspatially related to the laser rod.

Furthermore, stress birefringence is caused throughout the laser rod byuneven temperature, so that light polarized tangentially encounters adifferent index of refraction than light polarized radially at allpoints not on the axis. The total result of the varying indices ofrefraction is a difference in path length with both distance from theaxis and with polarization and a consequent reduction beam definition.

In short, pump-induced and cooling-induced indexof-refraction gradientsacross the diameter of a laser rod are very undesirable. These gradientsarise largely because of induced temperature gradients. However, forsingle-shot operation the temperature gradient during the pulse can bemade negligible by mode selecting as described above or by proper choiceof doping level and pumping geometry. For high-repetition-rateoperation, however, there is necessarily a radial temperature gradientsince heat is being introduced uniformly per unit volume but is onlyextracted through the outer surface. The magnitude of the thermalgradient depends on the thermal conductivity of the laser glass, themagnitude being high for glasses with low thermal conductivity.

in general, a radial thermal gradient will produce a radial gradient inthe index of refraction. In the case of laser glass, this index gradientis such that the index of refraction is higher in the center of the rodthan at its surface, resulting in a positive lens effect induced in therod. This occurs for rods having a length to diameter ratio (L/D) ofabout 1 or greater. For a MD of about 1 or less, the thermal gradient ispredominantly axial. However, the most efficient glass laser operationoccurs with a L/D of about 40: 1, so the thermal gradient associatedwith most lasers is predominantly radial. Also, in most lasers the endsof the rods are often masked from the pumping light, so that anypossible axial gradient is further reduced. For a L/D 40:1 laser rod, alens power of +8 diopter (D) has been measured in an 18 inch pumpedlength of a laser rod at an otherwise desirable operating point. In sucha case, the incident parallel laser light is focused to a point withinthe laser rod. The undesirability of such focusing is immediatelyapparent when it is considered that one of the advantages of lightgenerated by a laser is that such light should be collimated.

In connection with a solution to this problem it is known that thethermal conductivity of glass cannot be changed significantly. It isalso known that the index gradient resulting from a given temperaturegradient is a complicated function of the expansion coefficient, theindex of refraction, Poissons ratio, Youngs modulus and the stress-opticcoefficients.

The presence of induced positive lens power in a laser rod has thefollowing undesirable consequences:

1. The active laser volume is reduced, reducing the laser efficiency andincreasing the tendency toward damage,

2. The beam spread of the laser increases, which is undesirable becausethe maximum laser energy deliverable at a target is obtained with theminimum beam spread,

3. Self-focusing, normally seen in many materials under the influence ofa high-power laser beam, occurs at a much lower threshold when even asmall amount of positive lens power is initially present in thematerial. Self-focusing occurs because the index of refraction, n,

is expressible as n n n(E where n, is the usual index of refraction andn(E is a small change in :1 caused by the presence of the high intensitylaser electric Field E. Since n(E 0, and laser beams in general areGaussian-like in shape, i.e., with a higher intensity at beam centerthan at beam edge, the index of refraction will, in general, be higherat the center of the medium than at the edge. If MB) is high enough, thebeam collapses due to this dynamic positive lensing into adiffraction-limited filament. At this point the power density exceedsthe damage threshold of the material and a fine fossil record is left ofthe laser beam passage, and

4. Optical elements in the laser cavity can be damaged because of thereduced beam diameter caused by thermal lensing.

For the most part, the laser art is proceeding without compensation forthese problems although a number of approaches have been employed in anattempt to circum vent the thermal lensing problem.

One approach is to use rods of small cross-section as the laserablematerial. The disadvantages of this ap proach are that the obtainableoutput energy is too low and the beam spread is too high due todiffraction from the necessarily small aperture.

Another approach is to place a negative lens in the laser cavity. Ingeneral, however, lens compensation is impractical, especially for thestress birefringence, since a fixed lens becomes useless in the face ofa constantly changing variation in the indices of refraction; and,obviously, a series of insertable lenses are also unsatisfactory.

Another approach is to use curved end mirrors to offset thethermally-induced positive lens power of the reduced beam diametercaused by the thermal lensing.

Another technique is the placement of an afocal telescope inside thelaser cavity to reduce the effects of the thermally-induced lens poweron the damage threshold of the cavity elements.

In general, the disadvantages of using extra elements in the cavity arethat such elements can work for only one set of operating conditions,and that with long rods, a focus can occur within the rod, which cannotbe corrected with extra elements. Also the efficiency of the laser is,in general, reduced by these elements. Another disadvantage is that thelaser cavity cannot be aligned statically, i.e., the thermally-inducedpositive lens power must be present to balance the negative correctionin order to pass a collimated beam through the laser for alignmentpurposes. Of course, with extra elements in the cavity complexity, cost,alignment difficulties and damage probabilities all increase.

The foregoing approaches utilize various physical means to nullify oreliminate thermal gradients. The present invention utilizes a laserglass of special composition which nullifies the effect of thermalgradients. The use of compositions to nullify the effect of thermalgradients is not in and of itself novel. However, in all of the knowprior art laser devices which include a glass composition so selected tonullify the effect of thermal gradients various physical components wererequired along with the glass composition. For example, athermallystable compositions have been suggested for either radially polarizedlight or tangentially polarized light. For such compositions, physicalmeans for mode selecting to either radially polarized or tangentiallypolarized modes are required. Compositions have also been suggested fora laser which propogates both radially and tangentially polarized lightbut in this case a Faraday rotator is required in the cavity in order toaverage the radial and tangential distortions associated with the glass.It has been considered desirable to nullify the effects of thermalgradients solely with an appropriate composition of the laserablematerial without the necessity of rotators or mode selecting means. Thisis especially desirable in Q-switched systems utilizing a long rod andlow output reflectors since in such a system most of the energy build-upoccurs in the last pass through the rod, so that any form ofdiscrete-element, intracavity correction scheme is not capable ofcorrecting for the thermally-induced lens power in the rod.

This situation is especially acute with devices that average the radialand tangential distortions by alternating between these distortionsthrough the use of a Its-wave Faraday rotator within the cavity. Such ascheme is effective under long-pulse operation. However, the averagingeffect is not as successful in a highperformance Q-switched laser, wherethe change in pulse energy is very large for each pass through the rod.

In fact with Q-switched operations, an averaging scheme using a Iii-waveFaraday rotator is not totally effective in preventing self-focusing inthe rod because of the small residual thermally-induced positive lenspower in the rod for one polarization. When laser intensity (1. As isexplained below, except for Pockels glass which is a poor laser glass,it is not possible to correct for both polar components of polarizationsimultaneously. builds up in a few round-trip passes in the laser cavityand with the greatest accretion of energy occurring on the last passthrough the rod the residual positive lens characteristic which cannotbe prevented promotes self-focusing.

SUMMARY OF THE INVENTION In accordance with the present invention, aglass composition is utilized which enables discrimination between theradial and tangential polarization light vectors by athermalizing foronly one of these vectors while over-athermalizing for the other vector.When the glass is athermalized for radially polarized light thetangentially polarized light vectors are over-athermalized and arepropogated within the laser glass as if the laser glass were a negativelens. Thus, the selected polarization component which propogates withinthe negative lens has a tendency to miss the end reflectors in the lasercavity and pass outside the laser configuration The glass may also beathermalized for tangentially polarized light and over-athermalized forradially polarized light.

It is accordingly an object of the present invention to provide a meansfor compensating for the effects of thermal gradients and stressbirefringence produced in a laser rod by the heat from the flash tube.

It is an additional object of the present invention to compensate forthe effects of thermal gradients without the utilization of polarizationrotators or mode selecting means.

It is a further object of the invention to provide a laser device whichis athermally stable for all modes propogating within the laser cavity.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic illustrationof a prior art laser device showing focusing of the laser output when athermal gradient is present;

FIG. 2 is a diagrammatic illustration of a laser device in accordancewith the present invention utilizing a laser rod that is stable for theradial component of polariaztion and over-athermalized for thetangential component when a temperature gradient is present; and

FIG. 3 is a view similar to FIG. 2 illustrating an embodiment of theinvention-which is thermally stable for the tangential component.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is alaser device which is athermally stable even when operated at highrepetition rates. In general, laser devices are thermally unstable dueto gradients induced across the rod. These gradients arise largelybecause of induced temperature gradients but are also the result ofvarious other factors. For example, a non-uniform inversion will producea non-uniform index of refraction. This factor, however, is easilyeliminated by arranging the pump light uniformally along the lasermaterial so that there is equal inversion along any radius. Temperaturegradients can arise from non-uniform azimuthal heating, but by placingflashtubes symmetrically about the rod this non-uniform azimuthalheating can be avoided. How-ever, radial gradients in the rod areinevitable if reasonable pulse repetition rates and cooling of the rodoccur. Thus, even with the elimination of azimuthal gradients,distortion of the cavity nevertheless arises. Such distortions are dueto three factors. The change in temperature (AT) from center to edge ofthe rod leads to an expansion gradient in the rod due to the coefficientof expansion of the glass, causing the index of refraction to decreasewith increasing temperature. With a difference in temperature fromcenter to edge of the rod the index of refraction also differs fromcenter to edge because the change of index with temperature at constantdensity is positive for common laser glasses. Finally, thermal gradients(AT) within the glass produce stresses which result in both a change inindex of refraction and birefringence. For glasses, the change in indexof refraction with temperature increase, a,,, can be positive ornegative. Because of this fact the effects producing the index changecan be made to cancel each other by an appropriate glass compositionwith the correct negative value of a, which algebraically cancels theother positive factors.

Presently only one particular glass is known with such a negative 0:,which is athermally stable for both radial and tangential components ofpolarization. This glass is a glass developed by Pockels.( F. Pockels,Ann Physik, Vol. 9, p. 220 (1902); Vol. 11, p. 650 (1903) Unfortunately,the Pockels glass has been found to be a poor laser glass. Also, inorder to achieve the balance with both components of polarization at1.06u (the neodymium wavelength) the temperature must be lowered toapproximately dry ice temperature.

In accordance with the present invention, the laser glass is so selectedthat it is athermally stable for either radially polarized light ortangentially polarized light,

but not both. The composition is also over-athermaL ized for theparticular vector for which the glass is not thermally stable. Thus, acomposition which is athermally stable for radially polarized light isover-athermalized for tangentially polarized light and vice versa.

The following discussion, which appears in the article Glass Lasers byE. Snitzer in Applied Optics, Vol. 5, Number 10, October 1966, theteachings of which are incorporated herein by reference, demonstratesthat it is theoretically possible to athermalize a glass for eitherradially or tangentially polarized light.

For a solid rod of length L, the total optical path length P,(r) for atypical ray parallel to the axis and displaced a distance r from thecenter and with its plane of polarization in the radial direction isgiven by P,.(r)=nL{l+ [aJ-p/v (e +e )q/v 5] 2. where n is the index ofrefraction of the glass with no temperature gradient and no strain and Tis the difference in temperature between he center and points 'at adistance r from the center. The es are the strains in a cylindricalcoordinate system. The quantities q/v and p/v are the strain-opticcoefficients which relate the change in index of refraction to thestrains in the directions parallel and perpendicular, respectively, tothe plane of polarization of the light. The corresponding expression fortangential polarization is P, (r) "L {1+ [a,.T-p/v .1+ r) q/ 0 l- For anisotropic medium, the principle strains are related to the stresses 0'by where E is Youngs modulus and s Poissons ratio.

For a long rod with the end effects neglected, the problem is one ofplane strain. If the ends are free of traction so that they can move inresponse to the heating, the stresses are related to the thermaldistribution y where a is the linear expansion coefficient and where Fand R are defined by F a" f Trdr a being the radius of the rod. Thedependence of the optical path length on the temperature can be obtainedby substituting from (5) into (4) and in turn into (2) and (3). Thequantities desired are the differences in optical path lengths AP (r)and AP (r) between rays through the center and through points at r. Theresults are The difference between AP, and A]; gives the birefringence.If p/v and q/v are equal, as they are for a Pockels glass, there is nobirefringence. Furthermore, the only quantity which depends on thedetails of the temperature is R and it has a coefficient which goes tozero for a Pockels glass. It is interesting to note that the average ofthe radial and tangential components of AP is also independent of R.

The conditions for the quantities in the braces in (7) and (8) to beequal to zero can be regarded as a requirement on the a /a. This isbecause s, p/v and q/v do not depend strongly on the glass composition.Several glasses have been measured, including lead silicates,borosilicates, and alkaline earth silicates and it has been found thatPoissons ratio varies from 0.19 to 0.26. Furthermore, the quantities inbraces in (7) and (8) are not sensitively dependent on s. Pockelsmeasurements indicate p/v q/v 0.42 for a silicate glass containingapproximately 75 percent by weight of PbO. For the more common glassesthe strain-optic coefficients are smaller; q/v decreases more rapidlythan p/v. A light flint silicate (54.3 wt. percent SiO 33 PbO, 1.5 B 3NaO, 8 K 0) has the values p/v 0.306 and q/v 0.213, and for a borosilicatecrown (68.2 wt. percent SiO B 0 10 Na O, 9.5 K 0, 2 A1 0 p/v 0.269 andq/v 0.147.

To obtain an estimate of the required a to reduce the various APs tozero, typical values for the parameters are assigned. The strain-opticparameters are taken as p/v 0.3 and q/v 0.21 and Poisson's ratio as s0.25. A linear expansion coefficient of a= l0'" C is assumed. If thetemperature T varies as r, then R/T l/(p+2); for a quadratic dependenceof T on the radius, p 2 and R/T= V4. With these values for theparameters the braces in (7), (8) are equal to zero for a 42 Xl0"" C,34Xl0' C, 38Xl0' C, respectively. Note that the required values of a,are all negative.

From the foregoing it is clear that for a parabolic radial temperaturedistribution, the changes in pathlength can be written as:

Equations and 16 show that it is theoretically possible to reduce theeffects of thermal gradients by utilizing a glass composition with aproper; a which causes the glass to exhibit no appreciable change inoptical pathlength from center to edge of the rod for one polarizationwhen the temperature gradient is present while causing the difference inoptical pathlength from center to edge for the other polarization toincrease.

In addition to the foregoing theoretical proof, actual tests haveindicated that for a number of glasses, it is possible to athermalizefor light which is either radially or tangentially polarized, i.e.,AP,(r) 0 or AP U) 0, and that under these conditions light in the otherplane of polarization can have AP(r) 0. Thus, either one of a radiallyor tangentially polarized component of a beam can pass through a rodcontaining a radial temperature gradient and see no lens power while theremaining polarization can see a negative lens power. Accordingly, onlyeither radially or tangentially polarized beams, but not both, build upcavity oscillations. The non-oscillating tangentially or radiallypolarized component is refracted out of the cavity.

In order to illustrate the laser device of the present invention, atypical prior art laser device 10 is shown in FIG. I for comparison andcomprises a glass laser rod 12 optically aligned between reflectors l4and 16. Lightwaves making a single complete pass through laser rod 12are shown by arrows 18. Lightwaves 18 are parallel rays as they enterlaser rod 12 but when a tem perature gradient is present within rod 12as the light travels within the rod, the rays converge and focus,causing a loss of collimation.

The laser device of the present invention is shown in FIGS. 2 and 3 andcomprise a rod 20, 20 optically aligned in a resonant cavity formed byreflectors 22, 22 and 24, 24'. In connection with the laser cavity, itis to be understood that the invention is not intended to be limited tolaser oscillators. The laser rod of the present invention can beutilized in amplifying devices which do not include end reflectorsforming a cavity and in fact such amplifiers are athermally stable whenrods in accordance with this invention are utilized. With oscillatorsonce pumped by a suitable pump source (not shown) to an excited state,laser oscillations occur between reflectors 22, 22 and 24,24. The laserlight which results from laser rod 20 has two components ofpolarization, a tangential component and a radial component. In FIG. 2,the tangential component is represented by arrow 30 and the radialcomponent is represented by arrow 32. In FIG. 3, the tangentialcomponent is represented by arrow 30' and. the radial com ponent isrepresented by arrow 32'.

The device shown in FIG. 2 is athermalized for radially polarized lightand over-athermalized for tangentially polarized light. As a result,radially polarized light 32 oscillates and propagates within laser rod20 without appreciable bending, thereby providing a collimated output ofthis component of polarization. The composition is over-athermalized forthe tangential component of polarization so that the tangentialcomponent of polarization 30 which also propogates within laser rod 20is bent by the laser rod. Such bending causes this component ofpolarization to pass outside the laser cavity by being refracted throughthe edge of the glass as shown at 40 or by passing through the end ofthe rod as shown at 42 at such an angle that this component misses theend reflector 24. Such bending eliminates the non-collimated light rays.The device shown in FIG. 3 is identical in operation to the device shownin FIG. 2 except that the composition is chosen 9 to be athermallystable for tangentially polarized light and oversathermalized forradially polarized light.

In connection with the foregoing illustrations of the invention, it isto be understood that rectangular or slab laser rods as well as circularcylindrical laser rods can be athermalized in accordance with theinvention.

As was explained above, various glasses were tested. The composition inpercent by weight of representative examples of such glasses is given inTable I below:

The various glasses shown in Table I were formed into rods suitable forlaser applications and the rods were tested to determine the change inoptical pathlength from center to edge of the rod for both the radialand tangential components of polarization. The rods were measured forsuch a change in pathlength by passing a 1.06 micron gas laser beamthrough the rod and measuring the optical power. The change inpathlength from center to edge AP was determined from the measurement ofoptical power in accordance previously used in devices for applicationswhere thermal gradients are expected. Comparison of the measurements forExample 6 with the measurements for the other examples shown in Table [1reveals that Examples 7 and 8 exhibit improved thermal stability inaccordance with the teachings of the invention when incorporated in alaser device. As is shown in these Examples, the optical pathlengthdifference from center to edge for radially polarized light is verysmall when a temperature gradient is present and in fact approacheszero, thereby stabilizing this component of polarization. On the otherhand, the optical pathlength difference for the tangential component islarge in comparison with the radial component and is negative,indicating that the light diverges and thus passes from the laser cavityin accordance with the theory of the invention. The remaining Examples 15 also show excellent properties in accordance with the invention inthat the optical pathlength difference for one component approaches zerowhile rays of the other component diverge as propogated.

All of the glasses shown in Table II are laser glasses, that is, theycontain active ions which exhibit stimulated emission when excited by apump light source. When such glasses are formed into laser rods and therods are positioned along an optical axis within an opticallyregenerative cavity, light amplification of the stimulated emissionoccurs. With laser glass having properties approaching those disclosedin this specification, the output of laser light from the opticallyregenerative cavity will have less of a beam spread than previouslyknown laser devices.

The preparation of the glass of Example 7 is given below. The followingtotal amounts of batch constituents are added to a ceramic cruciblewhich is preheated to a temperature of 2700 F:

with the following relationship: P= (2 AP/R), where P 40 (kn-slime"!Weight in Grams is the optical power and R is the radius of the rod.

The results of the foregoing measurements are shown 81%0 2:30.30 K 31.60 1n Table 11 below. (N03 344% Ba(N0 2553.40 TABLE 11 T10, 470.40 ZnO319.20 Radial Tangential a, Per Degree Al(OH) 305.60 MeasurementMeasurement sb,o, 214.40 Example AP/ATl AP/A'Il Centigrade X 10- Nd o82,40

1 .727 x 10* -.351x 10* 32. g 2%; 1%: :32: 1g: 1 6 The constituents aremelted under a nitrogen at- 4 .815 x 10* .s9s x 10* -35 mosphere and areloaded in portions sufficient to allow 5 X for the expansion of gases.Crucible filling is accom- 6 .269x10- .144x10* 21.s 7 x l .1 x plishedin approximately 5 hours. After filling, the 8 -.s45 x 10* .1ss x 10*49.5 crucible is maintained at a temperature of 2700 F for about hourand then lowered to a temperature of 2400 F and maintained at thattemperature for about 2 hours. The temperature of the crucible isthereafter brought to 2300 F and maintained at that temperature forabout 15 hours. The temperature is decreased to 2200 F and maintained atthat temperature for about 1 hour and then increased to 2400 F andstirred for about 8 hours. Thereafter the temperature is lowered 50 Fand stirred for about 16 hours at 2350 F. The stirrer is removed and theglass allowed to cool to 2200 F, whereupon the glass is poured into agraphite mold which is preheated to 800 F. The cast glass is thenannealed for about 36 hours starting at 1030 F and is stopped at atemperature of about 300 F. The initial temperature drop during theannealing cycle is about 30 F per hour for the first few hours.

The other glasses shown in Table I can be prepared by a similarprocedure when an adjustment for the constituents is made so as tocorrespond to the composition of the final product.

Accordingly, by using glasses having the physical properties describedabove it is possible to utilize oscillators and amplifiers with largelaser glass components at high repetition rates without the problemsassociated with thermal gradients.

The invention rnay be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription, and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed is:

1. A laser apparatus for generating laser light in two polarizationcomponents, a first polarization component and a second polarizationcomponent, the laser apparatus comprising pumping means, a glass laserrod and an optically regenerative laser cavity defined by a pair ofreflectors, said glass laser rod being positioned between said pair ofreflectors in optical contact with said pumping means, said glass laserrod having a negative index of refraction for a temperature increase sothat thermal gradient factors including the coefficient of linearexpansion, the temperature coefficient of the index of refraction,Poissons ratio, and the stress-optical effects, algebraically tend tocancel each other out for light in the first polarization component soas to reduce the difference between the optical path length of the firstpolarization component in a ray through the center of said rod with theoptical path length for the first polarization component in a raypassing along said rod near the periphery thereof when a temperaturegradient caused by pumping said rod by said pumping means is presentwithin said rod from its center to its periphery, said negative index ofrefraction of said glass laser rod also being of a value so that thermalgradient factors algebraically combined to render the optical pathlength greater at the periphery of said rod than at the center of saidrod for light generated in the second polarization component, the laserrod being positioned within said laser cavity in relationship to saidreflectors so that light in the first polarization component strikes thereflectors while light in the second polarization component misses saidreflectors and passes from said laser cavity.

2. The apparatus as set forth in claim 1 wherein the first polarizationcomponent is radially polarized light.

3. The apparatus according to claim 1 wherein the second polarizationcomponent is tangentially polarized light.

4. A laser apparatus for generating laser light in two polarizationcomponents, a first polarization component and a second polarizationcomponent, the laser apparatus comprising pumping means, a glass laserrod and an optically regenerative laser cavity defined by a pair ofreflectors, said glass laser rod being positioned between said pair ofreflectors in optical contact with said pumping means, said glass laserrod having a negative index of refraction for a temperature increase sothat thermal gradient factors including the coefficient of linearexpansion, the temperature coefficient of the index of refraction,Poissons ratio, and the stress-optical effects, algebraically tend tocancel each other out for light in the first polarization component soas to reduce the difference between the optical path length of the firstpolarization component in a ray through the center of said rod with theoptical path length for the first polarization component in a raypassing along said rod near the periphery thereof when a temperaturegradient caused by pumping said rod by said pumping means is presentwithin said rod from its center to its periphery, said negative index ofrefraction of said glass laser rod also being of a value so that thermalgradient factors algebraically combine to render the optical path lengthgreater at the periphery of said rod than at the center of said rod forlight generated in the second polarization component, the laser rodbeing positioned within said laser cavity in relationship to saidreflectors so that light in the first polarization component strikes thereflectors while light in the second polarization component misses saidreflectors and passes from said laser cavity and wherein said glass rodis of approximately the following weight percent of constituents:

Slo 59.33 K 0 27.23 C30 8.] l SID- 0;, [.05 z a 2.43 SnO 1.95

5. A laser apparatus for generating laser light in two polarizationcomponents, a first polarization component and a second polarizationcomponent, the laser apparatus comprising pumping means, a glass laserrod and an optically regenerative laser cavity defined by a pair ofreflectors, said glass laser rod being positioned between said pair ofreflectors in optical contact with said pumping means, said glass laserrod having a negative index of refraction for a temperature increase sothat thermal gradient factors including the coefficient of linearexpansion, the temperature coefficient of the index of refraction,Poissons ratio, and the stress-optical effects, algebraically tend tocancel each other out for light in the first polarization component soas to reduce the difference between the optical path length of the firstpolarization component in a ray through the center of said rod with theoptical path length for the first polarization component in a raypassing along said rod near the periphery thereof when a temperaturegradient caused by pumping said rod by said pumping means is presentwithin said rod from its center to its periphery, said negative index ofrefraction of said glass laser rod also being of a value so that thermalgradient factors algebraically combine to render the optical path lengthgreater at the periphery of said rod than at the center of said rod forlight generated in the second polarization component, the laser rodbeing positioned within said laser cavity in relationship to saidreflectors so that light in the first polarization component strikes thereflectors while light in the second polarization component misses saidreflectors and passes from said laser cavity and wherein said glass rodis of approximately the following weight percent of constituents:

s10, 41.56 Na,o 14.42 BaO 26.74 s ,o, 0.85 Nap, 0.98 Nb,o, 15.45

6. A laser apparatus for generating laser light in two polarizationcomponents, a first polarization component and a second polarizationcomponent, the laser apparatus comprising pumping means, a glass laserrod and an optically regenerative laser cavity defined by a pair ofreflectors, said glass laser rod being positioned between said pair ofreflectors in optical contact with said pumping means, said glass laserrod having a negative index of refraction for a temperature increase sothat thermal gradient factors including the coefficient of linearexpansion, the temperature coefficient of the index of refraction,Poissons ratio, and the stress-optical effects, algebraically tend tocancel each other out for light in the first polarization component soas to reduce the difference between the optical path length of the firstpolarization component in a ray through the center of said rod with theoptical path length for the first polarization component in a raypassing along said rod near the periphery thereof when a temperaturegradient caused by pumping said rod by said pumping means is presentwithin said rod from its center to its periphery, said negative index ofrefraction of said glass laser rod also being of a value so that thermalgradient factors algebraically combined to render the optical pathlength greater at the periphery of said rod than at the center of saidrod for light generated in the second polarization component, the laserrod being positioned within said laser cavity in relationship to saidreflectors so that light in the first polarization component strikes thereflectors while light in the second polarization component misses saidreflectors and passes from said laser cavity and wherein said glass rodis of approximately the following weight percent of constituents:

SiO, 46.37 K 25.39 CaO 7.64 ZnO 4.42 A1 0, 2.75 Sb Q 1.1 1 TiO 6.62 Nd,05.67 Inert impurities 03 7. A laser apparatus for generating laser lightin two polarization components, a first polarization component and asecond polarization component, the laser apparatus comprising pumpingmeans, a glass laser rod and an optically regenerative laser cavitydefined by a pair of reflectors, said glass laser rod being positionedbetween said pair of reflectors in optical contact with said pumpingmeans, said glass laser rod having a negative index of refraction for atemperature increase so that thermal gradient factors including thecoefficient of linear expansion, the temperature coefficient of theindex of refraction, Poissons ratio, and the stress-optical effects,algebraically tend to cancel each other out for light in the firstpolarization component so as to reduce the difference between theoptical path length of the first polarization component in a ray throughthe center of said rod with the optical path length for the firstpolarization component in a ray passing along said rod near theperiphery thereof when a temperature gradient caused by pumping said rodby said pumping means is present within said rod from its center to itsperiphery, said negative index of refraction of said glass laser rodalso being of a value so that thermal gradient factors algebraicallycombine to render the optical path length greater at the periphery ofsaid rod than at the center of said rod for light generated in thesecond polarization component, the laser rod being positioned withinsaid laser cavity in relationship to said reflectors so that light inthe first polarization component strikes the reflectors while light inthe second polarization component misses said reflectors and passes fromsaid laser cavity and wherein said glass rod is of approximately thefollowing weight percent of constituents:

SiO 41.95 Na O 15.84 BaO 23.50 ZnO 4.12 A1 0:, 3.91 sb O 0.37 TiO, 5.1 lNd,0,, 5.16 Inert impurities .04

8. A laser apparatus for generating laser light in two polarizationcomponents, a first polarization component and a second polarizationcomponent, the laser apparatus comprising pumping means, a glass laserrod and an optically regenerative laser cavity defined by a pair ofreflectors, said glass laser rod being positioned between said pair ofreflectors in optical contact with said pumping means, said glass laserrod having a negative index of refraction for a temperature increase sothat thermal gradient factors including the coefficient of linearexpansion, the temperature coefficient of the index of refraction,Poissons ratio, and the stress-optical effects, algebraically tend tocancel each other out for light in the first polarization component soas to reduce the difference between the optical path length of the firstpolarization component in a ray through the center of said rod with theoptical path length for the first polarization component in a raypassing along said rod near the periphery thereof when a temperaturegradient caused by pumping said rod by said pumping means is presentwithin said rod from its center to its periphery, said negative index ofrefraction of said glass laser rod also being of a value so that thermalgradient factors algebraically combine to render the optical path lengthgreater at the periphery of said rod than at the center of said rod forlight generated in the second polarization component, the laser rodbeing positioned within said laser cavity in relationship to saidreflectors so that light in the first polarization component strikes thereflectors while light in the second polarization component misses saidreflectors and passes from said laser cavity and wherein said glass rodis of approximately the following weight percent of constituents:

s10, 40.93 K20 22.41 CaO 18.51 ZnO 3.90 A1203 2.43 sb,o, 0.98 no, 5.84Nd O 5.00

9. A laser apparatus for generating laser light in two polarizationcomponents, a first polarization component and a second polarizationcomponent, the laser apparatus comprising pumping means, a glass laserrod and an optically regenerative laser cavity defined by a pair ofreflectors, said glass laser rod being positioned between said pair ofreflectors in optical contact with said pumping means, said glass laserrod having a negative index of refraction for a temperature increase sothat thermal gradient factors including the coefficient of linearexpansion, the temperature coefficient of the index of refraction,Poissons ratio, and the stress-optical effects, algebraically tend tocancel each other out for light in the first polarization component soas to reduce the differencebetween the optical path length of the firstpolarization component in a ray through the center of said rod with theoptical path length for the first polarization component in a raypassing along said rod near the periphery thereof when a temperaturegradient caused by pumping said rod by said pumping means is presentwithin said rod from its center to its periphery, said negative index ofrefraction of said glass laser rod also being of a value so that thermalgradient factors algebraically combine to render the optical path lengthgreater at the periphery of said rod than at the center of said rod forlight generated in the second polarization component, the laser rodbeing positioned within said laser cavity in relationship to saidreflectors so that light in the first polarization component strikes thereflectors while light in the second polarization component misses saidreflectors and passes from said laser cavity and wherein said glass rodis of approximately the following weight percent of constituents:

s10 42.01 K 23.10 cio 18.81 ZnO 3.99 A1,o 2.50

1 ,0, 2.68 T10, 5.88 map, 1.03

10. A laser apparatus for generating laser light in two polarizationcomponents, a first polarization component and a second polarizationcomponent, the laser apparatus comprising pumping means, a glass laserrod and an optically regenerative laser cavity defined by a pair ofreflectors, said glass laser rod being positioned between said pair ofreflectors in optical contact with said pumping means, said glass laserrod having a negative index of refraction for a temperature increase sothat thermal gradient factors including the coefficient of linearexpansion, the temperature coefficient of the index of refraction,Poissons ratio, and the stress-optical effects, algebraically tend tocancel each other out for light in the first polarization component soas to reduce the difference between the optical path length of the firstpolarization component in a ray through the center of said rod with theoptical path length for the first polarization component in a raypassing along said rod near the periphery thereof when a temperaturegradient caused by pumping said rod by said pumping means is presentwithin said rod from its center to its periphery, said negative index ofrefraction of said glass laser rod also being of a value so that thermalgradient factors algebraically combine to render the o tical ath lengthgreater at the periphery of said rod t an a the center of said rod forlight generated in the second polarization component, the laser rodbeing positioned within said laser cavity in relationship to saidreflectors so that light in the first polarization component strikes thereflectors while light in the second polarization component misses saidreflectors and passes from said laser cavity and wherein said glass rodis of approximately the following weight percent of constituents:

510 39.13 Rb O 37.83 CaO 12.41 ZnO 3.30 mm, 2.06 sb,o 1.18 mm, 4.09

1. A laser apparatus for generating laser light in two polarizationcomponents, a first polarization component and a second polarizationcomponent, the laser apparatus comprising pumping means, a glass laserrod and an optically regenerative laser cavity defined by a pair ofreflectors, said glass laser rod being positioned between said pair ofreflectors in optical contact with said pumping means, said glass laserrod having a negative index of refraction for a temperature increase sothat thermal gradient factors including the coefficient of linearexpansion, the temperature coefficient of the index of refraction,Poisson''s ratio, and the stress-optical effects, algebraically tend tocancel each other out for light in the first polarization component soas to reduce the difference between the optical path length of the firstpolarization component in a ray through the center of said rod with theoptical path length for the first polarization component in a raypassing along said rod near the periphery thereof when a temperaturegradient caused by pumping said rod by said pumping means is presentwithin said rod from its center to its periphery, said negative index ofrefraction of said glass laser rod also being of a value so that thermalgradient factors algebraically combine to render the optical path lengthgreater at the periphery of said rod than at the center of said rod forlight generated in the second polarization component, the laser rodbeing positioned within said laser cavity in relationship to saidreflectors so that light in the first polarization component strikes thereflectors while light in the second polarization component misses saidreflectors and passes from said laser cavity.
 2. The apparatus as setforth in claim 1 wherein the first polarization component is radiallypolarized light.
 3. The apparatus according to claim 1 wherein thesecond polarization component is tangentially polarized light.
 4. Alaser apparatus for generating laser light in two polarizationcomponents, a first polarization component and a second polarizationcomponent, the laser apparatus comprising pumping means, a glass laserrod and an optically regenerative laser cavity defined by a pair ofreflectors, said glass laser rod being positioned between said pair ofreflectors in optical contact with said pumping means, said glass laserrod having a negative index of refraction for a temperature increase sothat thermal gradient factors including the coefficient of linearexpansion, the temperature coefficient of the index of refraction,Poisson''s ratio, and the stress-optical effects, algebraically tend tocancel each other out for light in the first polArization component soas to reduce the difference between the optical path length of the firstpolarization component in a ray through the center of said rod with theoptical path length for the first polarization component in a raypassing along said rod near the periphery thereof when a temperaturegradient caused by pumping said rod by said pumping means is presentwithin said rod from its center to its periphery, said negative index ofrefraction of said glass laser rod also being of a value so that thermalgradient factors algebraically combine to render the optical path lengthgreater at the periphery of said rod than at the center of said rod forlight generated in the second polarization component, the laser rodbeing positioned within said laser cavity in relationship to saidreflectors so that light in the first polarization component strikes thereflectors while light in the second polarization component misses saidreflectors and passes from said laser cavity and wherein said glass rodis of approximately the following weight percent of constituents: SiO259.33 K2O 27.23 CaO 8.11 Sb2O3 1.05 Nd2O3 2.43 SnO 1.95
 5. A laserapparatus for generating laser light in two polarization components, afirst polarization component and a second polarization component, thelaser apparatus comprising pumping means, a glass laser rod and anoptically regenerative laser cavity defined by a pair of reflectors,said glass laser rod being positioned between said pair of reflectors inoptical contact with said pumping means, said glass laser rod having anegative index of refraction for a temperature increase so that thermalgradient factors including the coefficient of linear expansion, thetemperature coefficient of the index of refraction, Poisson''s ratio,and the stress-optical effects, algebraically tend to cancel each otherout for light in the first polarization component so as to reduce thedifference between the optical path length of the first polarizationcomponent in a ray through the center of said rod with the optical pathlength for the first polarization component in a ray passing along saidrod near the periphery thereof when a temperature gradient caused bypumping said rod by said pumping means is present within said rod fromits center to its periphery, said negative index of refraction of saidglass laser rod also being of a value so that thermal gradient factorsalgebraically combine to render the optical path length greater at theperiphery of said rod than at the center of said rod for light generatedin the second polarization component, the laser rod being positionedwithin said laser cavity in relationship to said reflectors so thatlight in the first polarization component strikes the reflectors whilelight in the second polarization component misses said reflectors andpasses from said laser cavity and wherein said glass rod is ofapproximately the following weight percent of constituents: SiO2 41.56Na2O 14.42 BaO 26.74 Sb2O3 0.85 Nd2O3 0.98 Nb2O3 15.45
 6. A laserapparatus for generating laser light in two polarization components, afirst polarization component and a second polarization component, thelaser apparatus comprising pumping means, a glass laser rod and anoptically regenerative laser cavity defined by a pair of reflectors,said glass laser rod being positioned between said pair of reflectors inoptical contact with said pumping means, said glass laser rod having anegative index of refraction for a temperature increase so that thermalgradient factors including the coefficient of linear expansion, thetemperature coefficient of the index of refraction, Poisson''s ratio,and the stress-optical effects, algebraically tend to cancel each otherout for light in the first polarization component so as to reduce thedifference between the optical path length of the first polarizationcomponent in a ray through the center of said rod with the optical pathlength for the first polarization component in a ray passing along saidrod near the periphery thereof when a temperature gradient caused bypumping said rod by said pumping means is present within said rod fromits center to its periphery, said negative index of refraction of saidglass laser rod also being of a value so that thermal gradient factorsalgebraically combined to render the optical path length greater at theperiphery of said rod than at the center of said rod for light generatedin the second polarization component, the laser rod being positionedwithin said laser cavity in relationship to said reflectors so thatlight in the first polarization component strikes the reflectors whilelight in the second polarization component misses said reflectors andpasses from said laser cavity and wherein said glass rod is ofapproximately the following weight percent of constituents: SiO2 46.37K2O 25.39 CaO 7.64 ZnO 4.42 Al2O3 2.75 Sb3O3 1.11 TiO2 6.62 Nd2O3 5.67Inert impurities .03
 7. A laser apparatus for generating laser light intwo polarization components, a first polarization component and a secondpolarization component, the laser apparatus comprising pumping means, aglass laser rod and an optically regenerative laser cavity defined by apair of reflectors, said glass laser rod being positioned between saidpair of reflectors in optical contact with said pumping means, saidglass laser rod having a negative index of refraction for a temperatureincrease so that thermal gradient factors including the coefficient oflinear expansion, the temperature coefficient of the index ofrefraction, Poisson''s ratio, and the stress-optical effects,algebraically tend to cancel each other out for light in the firstpolarization component so as to reduce the difference between theoptical path length of the first polarization component in a ray throughthe center of said rod with the optical path length for the firstpolarization component in a ray passing along said rod near theperiphery thereof when a temperature gradient caused by pumping said rodby said pumping means is present within said rod from its center to itsperiphery, said negative index of refraction of said glass laser rodalso being of a value so that thermal gradient factors algebraicallycombine to render the optical path length greater at the periphery ofsaid rod than at the center of said rod for light generated in thesecond polarization component, the laser rod being positioned withinsaid laser cavity in relationship to said reflectors so that light inthe first polarization component strikes the reflectors while light inthe second polarization component misses said reflectors and passes fromsaid laser cavity and wherein said glass rod is of approximately thefollowing weight percent of constituents: SiO2 41.95 Na2O 15.84 BaO23.50 ZnO 4.12 Al2O3 3.91 Sb2O3 0.37 TiO2 5.11 Nd2O3 5.16 Inertimpurities .04
 8. A laser apparatus for generating laser light in twopolarization components, a first polarization component and a secondpolarization component, the laser apparatus comprising pumping means, aglass laser rod and an optically regenerative laser cavity defined by apair of reflectors, said glass laser rod being positioned between saidpair of reflectors in optical contact with said pumping means, saidglass laser rod having a negative index of refraction for a temperaturEincrease so that thermal gradient factors including the coefficient oflinear expansion, the temperature coefficient of the index ofrefraction, Poisson''s ratio, and the stress-optical effects,algebraically tend to cancel each other out for light in the firstpolarization component so as to reduce the difference between theoptical path length of the first polarization component in a ray throughthe center of said rod with the optical path length for the firstpolarization component in a ray passing along said rod near theperiphery thereof when a temperature gradient caused by pumping said rodby said pumping means is present within said rod from its center to itsperiphery, said negative index of refraction of said glass laser rodalso being of a value so that thermal gradient factors algebraicallycombine to render the optical path length greater at the periphery ofsaid rod than at the center of said rod for light generated in thesecond polarization component, the laser rod being positioned withinsaid laser cavity in relationship to said reflectors so that light inthe first polarization component strikes the reflectors while light inthe second polarization component misses said reflectors and passes fromsaid laser cavity and wherein said glass rod is of approximately thefollowing weight percent of constituents: SiO2 40.93 K2O 22.41 CaO 18.51ZnO 3.90 Al2O3 2.43 Sb2O3 0.98 TiO2 5.84 Nd2O3 5.00
 9. A laser apparatusfor generating laser light in two polarization components, a firstpolarization component and a second polarization component, the laserapparatus comprising pumping means, a glass laser rod and an opticallyregenerative laser cavity defined by a pair of reflectors, said glasslaser rod being positioned between said pair of reflectors in opticalcontact with said pumping means, said glass laser rod having a negativeindex of refraction for a temperature increase so that thermal gradientfactors including the coefficient of linear expansion, the temperaturecoefficient of the index of refraction, Poisson''s ratio, and thestress-optical effects, algebraically tend to cancel each other out forlight in the first polarization component so as to reduce the differencebetween the optical path length of the first polarization component in aray through the center of said rod with the optical path length for thefirst polarization component in a ray passing along said rod near theperiphery thereof when a temperature gradient caused by pumping said rodby said pumping means is present within said rod from its center to itsperiphery, said negative index of refraction of said glass laser rodalso being of a value so that thermal gradient factors algebraicallycombine to render the optical path length greater at the periphery ofsaid rod than at the center of said rod for light generated in thesecond polarization component, the laser rod being positioned withinsaid laser cavity in relationship to said reflectors so that light inthe first polarization component strikes the reflectors while light inthe second polarization component misses said reflectors and passes fromsaid laser cavity and wherein said glass rod is of approximately thefollowing weight percent of constituents: SiO2 42.01 K2O 23.10 CaO 18.81ZnO 3.99 Al2O3 2.50 Sb2O3 2.68 TiO2 5.88 Nd2O3 1.03
 10. A laserapparatus for generating laser light in two polarization components, afirst polarization component and a second polarization component, thelaser apparatus comprising pumping means, a glass laser rod and anoptically regenerative laser cavity defined by a pair of reflectors,said glass laser rod being pOsitioned between said pair of reflectors inoptical contact with said pumping means, said glass laser rod having anegative index of refraction for a temperature increase so that thermalgradient factors including the coefficient of linear expansion, thetemperature coefficient of the index of refraction, Poisson''s ratio,and the stress-optical effects, algebraically tend to cancel each otherout for light in the first polarization component so as to reduce thedifference between the optical path length of the first polarizationcomponent in a ray through the center of said rod with the optical pathlength for the first polarization component in a ray passing along saidrod near the periphery thereof when a temperature gradient caused bypumping said rod by said pumping means is present within said rod fromits center to its periphery, said negative index of refraction of saidglass laser rod also being of a value so that thermal gradient factorsalgebraically combine to render the optical path length greater at theperiphery of said rod than at the center of said rod for light generatedin the second polarization component, the laser rod being positionedwithin said laser cavity in relationship to said reflectors so thatlight in the first polarization component strikes the reflectors whilelight in the second polarization component misses said reflectors andpasses from said laser cavity and wherein said glass rod is ofapproximately the following weight percent of constituents: SiO2 39.13Rb2O 37.83 CaO 12.41 ZnO 3.30 Al2O3 2.06 Sb2O3 1.18 Nd2O3 4.09